1. Field of the Invention
The present invention relates generally to screen printing plates for forming a thin film having a prescribed pattern by transferring ink onto an object surface to be printed such as a semiconductor substrate and a glass substrate, and more specifically, to a screen printing plate for use in producing a very small pattern necessary to manufacture a printed circuit board, a semiconductor device, a display device, etc.
2. Description of the Related Art
Screen printing techniques are indispensable for manufacturing state of the art electronic parts such as a semiconductor device, and a flat display panel. In screen printing, a screen printing plate is used on which figures and patterns formed of opening portions and non-opening portions are formed mainly by means of photomechanical process. Ink is placed on the top surface of a screen printing plate. A squeegee is caused to slide across and press downward against the top surface of the screen on which the ink is placed, and in so doing presses ink onto the bottom surface of the screen through the opening portion of a pattern formed on the screen, and further transfer the ink onto the top surface of a material to be printed. Thus, a figure in accordance with the pattern formed on the screen is formed with the ink on the top surface of the material to be printed.
A screen printing plate used in screen printing includes a metal mask plate, a suspended metal plate, etc. These plates are manufactured by forming a figure and a pattern on a screen in a mesh form with an emulsifier, or by etching a thin metal (typically stainless) steel plate. Very thin lines for manufacturing electric parts, etc. are printed using a screen printing plate, with a so-called thin film printing technique. This is because such very thin lines can hardly be formed by means of so-called thick film printing techniques.
FIG. 1 is an enlarged perspective view showing a part of a general structure of the metal mask of a conventional screen printing plate. FIG. 1 is partially cut away for the purpose of illustration. Referring to FIG. 1, the metal mask 300 of the conventional screen printing plate includes a metal plate 301 having an opening area 306, and a non-opening area 307 formed in accordance with a pattern to be printed.
A number of openings 302 are formed in metal plate 301 at the positions of opening area 306.
At the time of printing, ink is placed over the top surface of metal mask 300. A squeegee is pressed downward and slid on the top surface of metal mask 300, and the ink 304 is pressed out to the bottom surface of metal plate 301 through openings 302, as shown in FIG. 2, to be transferred onto the top surface of a material to be printed 303 (for example a glass plate) and form a thin film.
When the thin film printing is conducted using a screen printing plate shown in FIGS. 1 and 2, however, the ink 304 squeezed out onto the bottom surface of openings 302 spreads over the top surface of glass plate 303. Referring to FIG. 2, it is usual for the ink 304 to spread from the edge of opening 302 for as far as about 50 .mu.m. A similar degree of spreading is observed both in the case of a metal mask and in the case of a mesh screen. Furthermore, the degree of spreading changes from position to position, and is never the same. With such spreading, clear printing can not be achieved because the edge of the ink of the thin film takes a wavelike form. Therefore, the very sophisticated printing necessary in a course of manufacturing printed wiring, a semiconductor device, a display device, etc. cannot accurately be obtained by using this conventional screen printing plate.
As a solution to this problem, one screen printing plate is disclosed in Japanese Patent Laying-Open No. 64 -87249 and appears at FIGS. 3 and 4 herein as screen printing plate 310 that includes a metal plate 317 having a number of openings 320 formed therein. Metal plate 317 is formed of nickel or the like. As shown in FIG. 3, a cross section 324 of the end of opening 320 takes a curved shape in the vicinity of the top surface of metal plate 317, and a straight line shape in the vicinity of the bottom surface. The periphery of opening 320 in the bottom surface of metal plate 317 is slightly higher (by about 0.5 .mu.m) than the bottom surface of metal plate 317, and its surface is flat. As a result, the edge of opening 320 toward the bottom surface of metal plate is sharpened. It is intended that this sharpened edge accurately form the end of ink to be printed.
Screen printing plate 310 shown in FIGS. 3 and 4 is manufactured as follows: Referring to FIG. 5A, a chromium coated substrate 313 is formed by evaporating a chromium film 312 on one surface of glass substrate 311 of a flat plate shape.
Referring to FIG. 5B, a number of openings 314 are formed in chromium coated substrate 313 in accordance with a prescribed rectangular pattern as shown in FIG. 5C by means of photolithography.
Chromium coated substrate 313 as shown in FIG. 5B is immersed in an Ni salt solution, for example, a luster nickel plating liquid, and a nickel film 316 is formed on chromium film 312 by electrolysis as shown in FIG. 5D. At that time, nickel does not precipitate in that portion in which glass substrate 311 is exposed except for the portion in contact with peripheral chromium film 312, and a part having no nickel film being formed thereon remains in this portion.
Referring to FIG. 5E, finally nickel film 316 is peeled off from chromium coated substrate 313. Thus, a screen printing plate having a number of openings is provided. At that time, there exists around each of the openings a part of nickel film formed directly in contact with glass substrate 311. The bottom surface of this part having grown in direct contact with the surface of the glass plate extends downwardly by the amount of the thickness of chromium film 312 from the bottom surface of nickel film 316 in the other parts. The thickness is about 0.5 .mu.m. The bottom surface of this part is kept from growing by the surface of glass substrate 311 and is flat. More specifically, the raised portion in the circumference of the opening is unavoidably formed by employing the manufacturing method shown in FIGS. 5A-5E.
Other problems yet to be solved are still encountered with the screen printing plate disclosed in Japanese Patent Laying-Open No. 64-87249. A first problem is that the technique disclosed in Japanese Patent Laying-Open No. 64-87249 still leaves room for improvements in the precision of printing results. For example, the applicant manufactured a screen printing plate as disclosed in Japanese Patent Laying-Open No. 64-87249 and printed a material having a dimension of 120 mm at one side, and the result revealed that an error of the end checked by the applicant was more than .+-.50 .mu.m. A second problem is that the technique disclosed in Japanese Patent Laying-Open No. 64-87249 does not allow printing of a long line. This is because nickel film 316 which is the body of screen printing plate is thin, and the strength of the screen printing plate is not enough to form a long opening for printing a long line. A third problem is that the thick film printing can not be conducted using the screen printing plate disclosed in Japanese Patent Laying-Open No. 64-87249. This is because nickel film 316 is formed by electrolysis, and, therefore, nickel film 316 can not be formed thicker than a certain degree.